The present invention relates to a semiconductor laser, and particularly to a semiconductor laser which is fabricated by an epitaxial growth method which is based upon a nonequilibrium growth mechanism such as metal-organic chemical vapor deposition (hereinafter referred to as MOCVD method), molecular beam epitaxy (hereinafter referred to as MBE method) or the like.
It is essential for a semiconductor laser to be provided with structure to stabilize a transverse mode. Prior art semiconductor lasers prepared by liquid phase epitaxy, such as a CSP (channeled substrate planar) laser are known, in which a substrate is provided with a groove stripe and a planar active layer is fabricated by utilizing a growth mechanism which is specific to the liquid epitaxy. However, if crystals are grown on the substrate provided with the groove stripe by an epitaxial growth method which is based upon a nonequilibrium growth mechanism such as a MOCVD method or a MBE method, the active layer is bent by the specific growth mechanism which maintains the shape of the substrate. Therefore, the light guiding effect achieved by the CSP laser is not expected. Further, lattice defects are formed in the active layer which results in deterioration of reliability. Therefore, it is known to grow the crystal such that a planar active layer is fabricated, and then to provide another layer on the active layer to stabilize the transverse mode. An example thereof is shown in FIG. 1. This laser structure has been disclosed by J. J. Coleman et al., Applied Physics Letters, 1980, Vol. 37, p. 262.
According to this method of fabricating the laser, there are successively formed an n-GaAs buffer layer 2, an n-GaAlAs cladding layer 3, an active layer 4, a p-GaAlAs cladding layer 5, and an n-GaAs current blocking layer 6 on an n-GaAs substrate 1 by MOCVD method. Then, the upper current blocking layer 6 in which the laser oscillation takes place, is removed by photolithography and chemical etching. Thereafter, a p-GaAlAs cladding layer 7 and a p-GaAs cap layer 8 are grown by MOCVD method.
Next, an n electrode 9 and a p electrode 10 are formed. According to this structure, the current injection is restricted within the radiation region by the current blocking layer 6, and the fundamental transverse mode is realized due to light absorption of the current blocking layer 6 outside the stripe region. However, since the surface of the p-GaAlAs cladding layer 5 has been oxidized, the growth of the further layers will result in abnormal layer formation at the interface and dislocation lines 11 are formed due to dependence on the orientation facet of the crystal that is grown. Therefore, the laser characteristics are deteriorated and the reliability is lowered.
Further, according to Kuroda et al., preparatory documents for the National Convention of the Association of Electronic Communications, Department of Optical-Electromagnetic Waves, 1984, Vol. 2, p. 23, there is disclosed a method of stabilizing the transverse mode by providing a multi-quantum well (MQW) layer as an optical guide layer on the active layer, and by restricting the radiation region by disordering both sides of the MQW layer by the diffusion of impurities. However, this method involves technical difficulty with respect to limiting the depth of diffusion within a range of the thickness of the cladding layer that is in contact with the active layer.